The example in the Reporting module is incorrect as the data does not match between the list report and summary report. The list report shows 3 Boston and 3 Atlanta office locations. The summarized report shows 4 Atlanta and 2 Boston.
Great spot! The summary is actually based on the Manager, hence Harry=4, Fred=2. The Office column should not have been shown as it does not make sense. In reality, if you also summarise on the Office column, you will see a further breakdown of rows to group based on Office.